This invention relates to the production of functional and polymerizable coumarin dyes for the preparation of side-chain coumarin dye-substituted polymers which exhibit nonlinear optical (NLO) properties. More particularly, the present invention is directed to the provision of functional coumarin dyes, polymerizable coumarin dyes and side-chain coumarin dye-containing polymers useful for the fabrication of organic polymeric nonlinear optical materials.
The field of nonlinear optics is concerned with the interactions of electromagnetic fields with materials to produce new fields that are altered in phase, frequency, amplitude or other propagation characteristics from the incident field. See, for example, Y. R. Shen, "The Principles of Nonlinear Optics," John Wiley & Sons, N.Y., 1984. The best known nonlinear optical effect is second harmonic generation (SHG) or frequency doubling. Optically nonlinear materials are used in frequency doublers for lasers, optical communications and computing equipment, laser resistance devices, and in opto-electronic devices for other applications. See, for example, G. Boyd, J. Opt. Soc. of Am. B, 6(4) 685 (1989).
Nonlinear optical devices such as frequency doublers have been based almost exclusively on inorganic materials, e.g. lithium niobate (LiNbO.sub.3) and potassium dihydrogen phosphate (KDP). Crystalline organic materials, e.g. methylnitroaniline (MNA), are also under development. The disadvantages of these materials include relatively poor laser damage resistance and difficulties in preparation and fabrication into opto-electronic devices. Additionally, single organic crystals are brittle and difficult to grow.
Organic polymeric materials with large delocalized pi-electron systems exhibit very fast NLO responses, have large optical nonlinearities, and the chemical synthesis of these materials can be altered to optimize their desirable physical characteristics while preserving their NLO properties. See, for example, D. J. Williams, Agnew. Chem. Int. Ed. Engl., 23, 690 (1984). Polymeric NLO materials can have very good mechanical properties. They can be mechanically tough and easily fabricated or processed into thin film geometries that are very desirable for integration with microelectronics. See, for example, G. H. Cross, et al., "Polymeric integrated electro-optic modulators," Proceedings of the SPIE, 1177, 79 (1989).
Two basic approaches exist for the synthesis of optically nonlinear polymers. One approach is to prepare guest-host materials by simply dissolving polarizable moieties (chromophores or dyes) as the guest in a polymeric host. This physical or solid solution may be severely limited in concentration of the chromophore due to limited solubility of the dye molecule. The other approach is to synthesize polymers that have chromophores chemically attached as either side-chain or main-chain substituents. These dye-substituted polymers have several distinct advantages over guest-host materials including higher limiting concentrations of the chromophore, reduced mobility and enhanced orientational stability of the chromophore, and improved optical, thermal and mechanical properties. See, for example, K. D. Singer, et al., Appl. Phys. Lett., 53(19), 1800 (1988).
The methods used to synthesize dye-substituted polymers each have their separate advantages and disadvantages which should be considered. The attachment or substitution of chromophores onto preformed polymers is often complicated by limited reactivity of the chromophores and/or the polymer and/or poor solubility of the chromophore in the polymer. Steric interference or blocking of reactive sites on the polymer can also occur once some dye molecules have been attached to the polymer. The result is often poor control over the extend of reaction and less effective substitution of the polymer with chromophores. The polymerization of dye-substituted monomers is sometimes inhibited by side reactions, for example, in free radical polymerization the growing chain end may terminate by reaction with the dye.
The nonlinear optical properties of crystalline coumarin dyes have been previously reported and described, e.g. in U.S. Pat. No. 3,858,124. Thus, it has been disclosed that some coumarin dye single crystals have significant NLO properties, and that one specific example (7-diethylamino-4-methylcoumarin) showed frequency doubling ability that was at least as good as lithium niobate (LiNbO.sub.3). This crystalline coumarin also showed better laser damage resistance than LiNbO.sub.3. These reports also documented one major problem of coumarins that is common to many other crystalline materials used in NLO applications, that of difficulty in growing single crystals of sufficient size and high quality for evaluation. Further, nonpolymeric coumarin dye NLO materials, such as 7-dimethylamino-4-methylcoumarin, are limited in their usefulness by having poor mechanical properties.
Dye-containing polymers for NLO applications have been reported. See for example U.S. Pat. Nos. 4,795,664; 4,779,961; 4,755,574 and 4,579,915. However, to applicants' knowledge, no coumarin dye-containing polymers having nonlinear optical properties have to date been produced or reported.
One object of the invention accordingly is the provision of novel side-chain coumarin dye-containing polymers having nonlinear optical properties.
Another object is to provide side-chain coumarin in dye-containing polymers which also have good thermal, mechanical and optical properties.
A still further object is the provision of functional coumarin dyes and polymerizable coumarin dyes for the synthesis of the above coumarin dye-containing polymers having nonlinear optical properties.
Yet another object is to provide coumarin dyes which have functional hydroxyl groups or which are chemically attached to vinyl monomers for use in producing coumarin dye-containing polymers having NLO characteristics.